tcp: add declarations to avoid warnings

Remove these errors:

net/ipv6/tcp_ipv6.c:1550:29: warning: symbol 'tcp_v6_rcv' was not declared. Should it be static?
net/ipv6/tcp_ipv6.c:1770:30: warning: symbol 'tcp_v6_early_demux' was not declared. Should it be static?

net/ipv6/tcp_ipv6.c:1550:29: warning: no previous prototype for 'tcp_v6_rcv' [-Wmissing-prototypes]
 1550 | INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
      |                             ^~~~~~~~~~
net/ipv6/tcp_ipv6.c:1770:30: warning: no previous prototype for 'tcp_v6_early_demux' [-Wmissing-prototypes]
 1770 | INDIRECT_CALLABLE_SCOPE void tcp_v6_early_demux(struct sk_buff *skb)
      |                              ^~~~~~~~~~~~~~~~~~
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -934,6 +934,8 @@ static inline int tcp_v6_sdif(const struct sk_buff *skb)
 }
 
 INDIRECT_CALLABLE_DECLARE(void tcp_v6_send_check(struct sock *sk, struct sk_buff *skb));
+INDIRECT_CALLABLE_DECLARE(int tcp_v6_rcv(struct sk_buff *skb));
+INDIRECT_CALLABLE_DECLARE(void tcp_v6_early_demux(struct sk_buff *skb));
 
 #endif